A.2 The Options field

A.2 The Options field

The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all link state advertisements. The Options
field enables OSPF routers to support (or not support) optional
capabilities, and to communicate their capability level to other
OSPF routers. Through this mechanism routers of differing
capabilities can be mixed within an OSPF routing domain.

When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain link state advertisements
to a neighbor because of its reduced functionality. Lastly, listing
capabilities in link state advertisements allows routers to route
traffic around reduced functionality routers, by excluding them from
parts of the routing table calculation.

Two capabilities are currently defined. For each capability, the
effect of the capability's appearance (or lack of appearance) in
Hello packets, Database Description packets and link state
advertisements is specified below. For example, the
ExternalRoutingCapability (below called the E-bit) has meaning only
in OSPF Hello Packets. Routers should reset (i.e. clear) the
unassigned part of the capability field when sending Hello packets
or Database Description packets and when originating link state
advertisements.

Additional capabilities may be assigned in the future. Routers
encountering unrecognized capabilities in received Hello Packets,
Database Description packets or link state advertisements should
ignore the capability and process the packet/advertisement normally.

This describes the router's TOS capability. If the T-bit is
reset, then the router supports only a single TOS (TOS 0). Such
a router is also said to be incapable of TOS-routing, and
elsewhere in this document referred to as a TOS-0-only router.
The absence of the T-bit in a router links advertisement causes
the router to be skipped when building a non-zero TOS shortest-
path tree (see Section 16.9). In other words, routers incapable
of TOS routing will be avoided as much as possible when
forwarding data traffic requesting a non-zero TOS. The absence
of the T-bit in a summary link advertisement or an AS external
link advertisement indicates that the advertisement is
describing a TOS 0 route only (and not routes for non-zero TOS).

E-bit

This bit reflects the associated area's
ExternalRoutingCapability. AS external link advertisements are
not flooded into/through OSPF stub areas (see Section 3.6). The
E-bit ensures that all members of a stub area agree on that
area's configuration. The E-bit is meaningful only in OSPF
Hello packets. When the E-bit is reset in the Hello packet sent
out a particular interface, it means that the router will
neither send nor receive AS external link state advertisements
on that interface (in other words, the interface connects to a
stub area). Two routers will not become neighbors unless they
agree on the state of the E-bit.